This invention relates to longitudinal mode laser diodes and, more particularly, to achieving higher yields of devises lasing at a desired wavelength.
Existing semiconductor laser fabrication processes have difficulty in achieving devices that oscillate at a precise wavelength. One of the contributing factors is a variation of the material gain function that may arise during epitaxial crystal growth. Typical variation in lasing wavelength of lasers made from different growths may amount to +/xe2x88x924 nm. However many applications require lasers to have a wavelength accuracy of +/xe2x88x921 nm or better, e.g., for the optical pumping of Nd:YAG lasers at 808 nm. Other factors such as stress, temperature and thermal non-uniformities often cause additional variable shifts. The mismatch between the achievable wavelength variation and end-user specifications has led to difficulties in achieving desirable manufacturing yields.
The most common type of laser diode structure is the double heterostructure which uses a ternary or quaternary material such as AlGaAs or GaInAsP in which a narrow bandgap, optically active, thin layer (0.1-0.2 xcexcm thick) is sandwiched between a pair of thicker, wider-bandgap cladding layers. A semiconductor, edge-emitting (longitudinal mode) diode laser typically includes a resonator formed of a solid state laser gain medium extending longitudinally between input and output mirror surfaces usually formed by cleaving. The distance between the facets defines a Fabry-Perot cavity which is capable of sustaining several different longitudinal lasing modes. The gain medium is typically enclosed on laterally adjacent sides by reflective material having an index of refraction n2 which is greater than the index of refraction n1 of the gain medium material.
To improve wavelength stability, feedback can be provided by locating a first or second order grating of suitable pitch either internally to the gain region of the solid state structure (for the DFB laser), or externally thereto (for the DBR laser). For example, U.S. Pat. No. 4,178,604 issued Dec. 11, 1979 shows a laser diode operating at 0.875 xcexcm stabilized by a first order grating having a pitch xcex9=0.123 xcexcm and an index of refraction n≅3.55 located between the active layer (n=3.6) and another layer (n=3.36). The grating is formed by using ultraviolet light to photolithographically etching a semiconductor layer located 0.3 xcexcm away from the 0.1 xcexcm thick active layer and having a refractive index lower than that of the active layer using interfering ultra-violet beams. The grating can be termed a xe2x80x9cfirst orderxe2x80x9d grating since the pitch xcex9=0.123 xcexcm is related to the desired laser wavelength xcex=0.875 xcexcm by xcex9=xcex/2n. Unfortunately, ultraviolet light photolithography requires extensive measure to prevent unwanted carbon coating of optics due to photolyzation of organic material which generally dictates that the process be performed in an extensively purged environment. Moreover, the surface of the mirror used to reflect the interfering waves must be extremely smooth to avoid unwanted scattering that would detract from the precise exposure of the photoresist, scattering being proportional to the inverse-fourth power of the wavelength of light employed. Finally, UV lasers are notoriously unreliable. It would be extremely desirable to avoid the use of ultraviolet light in the manufacturing process.
Another example of a DFB laser appears in vol. 18 Electronics Letters for Jan. 7, 1982, at pp. 27, 28 which shows a GalnAsP/InP laser operating at a wavelength of 1.5 xcexcm using a second order grating (xcex9=0.4522xcexc) etched into an n-lnP substrate. The grating was buried in a heterostructure comprised of a 0.17 xcexcm thick waveguide layer of Sn-doped, n-GalnAsP adjacent to a non-doped, 0.19 xcexcm thick GalnAsP active layer. To suppress unwanted Fabry-Perot, modes the rear facet of the cavity was inclined.
U.S. Pat. No. 4,704,720 issued No. 3, 1987 asserted that the grating used in the aforementioned laser was located too far away from (i.e, too weakly coupled to) the optical field so that, at certain values of the injected current, oscillation at the unwanted Fabry Perot modes occurred instead of at the desired single wavelength. Accordingly, the ""720 patent laser, operating at an exemplary wavelength of 1.3 xcexcm, located its second order grating (having a pitch of xcex9=0.4xcexc) in the strongest part of the optical field to obtain oscillation in a single longitudinal mode. Alternatively, a first order grating having a finer pitch (xcex9=0.2xcexc) was suggested.
While a second order grating can be produced using lower energy blue light beams in the photolithographic process, second order gratings located in the high intensity optical field of the active layer or of the waveguide layer give rise to diffraction orders that sap energy from the single desired longitudinal mode. It would, however, facilitate ease of manufacture if blue light beams could be used to produce second order gratings on wafers made from production runs of crystal growths having reasonable variation in their material gain function without incurring the penalty of exciting transverse modes that sap energy from the longitudinal beam, and especially at a wavelength of 0.808 xcexcm required to drive erbium doped fiber systems.
In accordance with the principles of the present invention, in one illustrative embodiment thereof, a wafer having a material gain function that would preferentially support an F-P laser mode at an unwanted wavelength xcex2 is provided with a second order dielectric grating located sufficiently remotely from the high intensity optical field to receive just enough transverse mode energy to provide feedback to reduce the gain at xcex2 and support oscillation at a desired wavelength xcex1. More particularly, it has been discovered that by providing a gain discrimination factor with order of magnitude xcex1≈0.1 cmxe2x88x921, the fraction of power lost to transverse mode radiation can be held to less than 1 percent which is sufficient to provide stabilizing feedback without sapping too much energy from the longitudinal beam. The grating, defined by effecting a corrugated interface between two layers in the epitaxial growth structure which are grown with slightly different indices of refraction, extends over the entire surface of the wafer. When the material is processed into Fabry-Perot laser devices, the coated facets provide most of the feedback while the grating alters the Fabry-Perot longitudinal mode spectrum to create a preferred resonance condition.